Gradation of a Developed Photographic Image. 309 



Thus :— 





Abscissa. 



Star. 



Scale. 



1 - 



2-65 (ordinate 155) 



4 = 



7 • 60 (ordinate 42) 



3 = 



4-95 



or 1 = 



1-65 



That is to say, the gradation of the plate when subjected to the red 

 light is much steeper than when subjected to the light of the amyl 

 acetate, and that to produce the same slope the ratio of the times of 

 exposure to red light would have to be shortened in the ratio of 1 : 1 *70 ; 

 that is, if the exposure Was doubled for the red light on each small 

 space ; then to make the slope the same for the amyl-acetate light the 

 successive exposures given with it would have to be 3*3 times. It must 

 be recollected that the first exposures required to give any deposit on a 

 plate would be widely different, being far larger for the red light. 



Results of Measures made. 



To avoid any white light with which the prisms were illuminated 

 reaching the plate through the slits, the following absorbing media 

 were placed in front of the slit at the places indicated. The times 

 of exposure are also shown. 



Scale No. 



Exposure. 



Absorbing medium in front 

 of slit. 



56-7 



65 min. 



Stained red glass. 



54-4 



20 „ 



)5 J) 



52 1 



5 „ 



>> >; 



50-6 



5 .„ 



Orange. 



47-5 



3 „ 



Lemon yellow. 



42-9 



2 „ 



Chrome green. 



38-3 



2 



Peacock green. 



55 55 



33-7 



10 sees. 



29-1 



8i „ 



Blue dye. 



Cobalt glass and blue dye. 



26-8 



12 „ 



22 2 



5 ,, 



17-6 



5 ,, 



Metliyl violet'.' 



8-4 



4 „ 



The following tables give the measured curves, and from them the 

 gradations are found, as in the above example, the exposures given 

 being as follows : — • 



